This invention relates to video signal recording and, more particularly, to digital video recording apparatus of compact size for recording digital video signals in standard and long play modes.
Digital signal recording apparatus, such as digital video tape recorders (DVTRs) have been developed for broadcasting purposes and result in the reproduction of video pictures having excellent quality. DVTR systems have been developed with two distinct formats: the so-called component digital recorder, known as the D-1 format, and the so-called composite type digital recorder, known as the D-2 format.
The D-1, component type format, records a luminance component and two color difference components by converting these components from analog to digital form. The luminance component is digitized with a sampling frequency of 13.5 MHz and each color difference component is digitized with a sampling frequency of 6.75 MHz. Since the ratio of the sampling frequencies of the luminance and color difference components is 4:2:2, the D-1 video recording technique also is referred to as the 4:2:2 technique.
When recording color video signals in the D-2 format, a composite color video signal is digitized with a samplying frequency that is four times the frequency f.sub.sc of the chrominance subcarrier. The digitized composite color video signal of the sampling frequency 4f.sub.sc then is processed and recorded.
The emphasis of digital video recording systems has been on enhancing the video picture reproduced therefrom. To this effect, each video signal sample is represented as an 8-bit digital signal, and these 8-bit samples typically are recorded without data compression. As a consequence, the quality of the recorded digital video information is quite high, but a typical video picture is represented by an extraordinary amount of information. Hence, a large quantity of record medium is needed for such high quality digital recording.
As an example of the amount of data needed to represent a digital video signal, reference is made to the D-1 format. When 8-bit data samples are produced by sampling the luminance and color difference components with the aforementioned sampling frequencies of 13.5 MHz and 6.75 MHz, respectively, the amount of information used to represent the video signal is about 216 MB/sec. This quantity of data can be reduced by omitting horizontal and vertical blanking periods and by sampling only those raster line intervals which contain useful video information. If 720 luminance pixels, 360 first color difference pixels and 360 second color difference pixels are sampled in each horizontal period, and if 250 lines per field contain useful video information, then the amount of data D.sub.v needed to represent an NTSC field in, the D-1 format, without data compression, is calculated to be: EQU D.sub.v =(720+360+360).times.8.times.250.times.60=172.8 MB/sec.
Similarly, if the D-1 format is used to record PAL video signals, the number of line intervals per field which contain useful video information is 300 and the field repetition rate is 50 per second, resulting in a data amount D.sub.v of: EQU D.sub.v =(720+360+360).times.8.times.300.times.50=172.8 MB/sec.
As is typical for digital recording, redundant data normally is added to the useful data for the purpose of error correction, and still additional data is added for formatting purposes. This increases the amount of data needed to represent video signals in the D-1 format to about 205.8 MB/sec.
As is known, audio information is digitized and recorded in the same track as the video information in the D-1 format. Typically, the amount of audio data D.sub.a that is recorded is on the order of about 12.8 MB/sec. Moreover, a typical track of digital data recorded in the D-1 format includes preamble and postamble data as well as additional data and margin for use in editing purposes. The amount of such additional data D.sub.o is on the order of about 6.6 MB/sec. Thus, even if the redundant data typically used for error correction and formatting is omitted, the amount of D.sub.t needed for the D-1 DVTR format is calculated as: EQU D.sub.t =D.sub.v +D.sub.a +D.sub.o =172.8+12.8+6.6=192.2 MB/sec.
When recording this amount of data D.sub.t in the D-1 format on video tape, one field of video information in the NTSC system is recorded in ten tracks and one field of video information in the PAL system is recorded in twelve tracks.
The recording tape normally used with digital video recorders is 19 mm wide. Typically, such video recording tape admits of two thicknesses: 13 .mu.m and 16 .mu.m. Such video recording tape is housed in a cassette and, depending upon the length of tape stored therein, these cassettes are known as large cassettes (L), medium cassettes (M) and small cassettes (S). Heretofore, the storage density for recording data in the D-1 format has been on the order of about 20.4 .mu.m.sup.2 /bit. If the storage density is increased, that is, if the recording area assigned to each bit on the magnetic tape is reduced, errors in the reproduced data tend to increase because of intersymbol interference, waveform deterioration caused by nonlinearities in the electromagnetic conversion (or interface), and the like. Even if error correction codes are used for recording, the storage density generally has not been able to be increased beyond 20.4 .mu.m.sup.2 /bit.
In accordance with the foregoing parameters, namely bit storage density, the amount of data used for recording in the D-1 format and usual tape transport speed, the typical recording capacity (in terms of recording time) on video tape having a thickness of 13 .mu.m in cassettes of different sizes is as follows:
______________________________________ S cassette 13 minutes M cassette 42 minutes L cassette 94 minutes ______________________________________
The typical recording capacities for these cassettes if the tape thickness is 16 .mu.m are as follows:
______________________________________ S cassette 11 minutes M cassette 34 minutes L cassette 76 minutes ______________________________________
Thus, when recording video information in the D-1 format, the maximum recording capacity for video tape that is 19 mm wide and that is housed in the L cassette is only about 11/2 hours. Notwithstanding the excellent picture quality that is produced from the DVTR, the recording capacity of even the largest D-1 cassette is not acceptable for consumer use. Although this limited recording capacity is satisfactory for broadcast purposes, it simply is too small for home use.
In contrast to digital video recording systems, conventional analog systems, such as Beta, VHS and 8-mm, admit of practical consumer use because they exhibit sufficient recording capacities of at least 2 hours or more. The quality of the video picture reproduced from such analog VTRs generally is quite good. However, when copies of an analog video tape are made, the rerecording of such analog video signals or the editing/dubbing thereof gradually degrades the quality of the video picture which eventually is reproduced. After several re-recordings, editings and/or dubbings, the quality of the video picture is so poor that it is not easily perceived by a user.
To overcome this problem, a digital video recorder has been proposed for use with magnetic tape that is 8 mm wide, or even narrower. Video data is compressed in a format that reduces distortion, increases recording density and effectively increases recording capacity.
As is known to most users of VTR systems, a conventional analog VTR, such as the 8 mm system, exhibits at least two recording modes, referred to as standard play (SP) and long play (LP) modes. The speed at which the video tape is transported in a standard play (SP) mode is greater than the tape transport speed in an LP mode. Thus, for a given time duration, less magnetic tape is consumed when video signals are recorded in the LP mode than in the SP mode. If a user wishes to record a broadcasted television program, one or the other of these modes may be selected, depending upon the duration of that program and the length of magnetic tape which remains available for recording. This selectability between SP and LP modes, although commonly found in analog VTR systems, is not used in digital VTR systems, even though this function would be useful in a digital VTR.
Like most VTR systems, the conventional 8 mm system uses rotary heads for recording and reproducing information, both in the SP and LP modes. By reason of the faster tape transport speed in the SP Node, the pitch of the tracks recorded by the rotary heads is 20.5 .mu.m and the pitch of the tracks recorded in the LP mode is 10.25 .mu.m. The width of the recording/reproducing head, or stated otherwise, the gap length of the head, is 15 .mu.m, thus resulting in a guard band of 5.5 .mu.m between adjacent tracks when information is recorded by these heads in the SP mode. A rotary erase head is used to provide an appropriate guard band, and the presence of one or more erase heads adds to the complexity and size of the VTR.
In both analog and digital VTRs, rotary transformers are used to couple electrical signals between the transducers and the processing circuitry. Rotary transformers generally are characterized as opposite type or concentric type transformers. In the opposite type rotary transformer, the rotor and stator are disposed opposite each other across a gap. In the concentric type of rotary transformer, the rotor and stator are disposed concentrically of each other. When the number of recording channels increases, as when a larger number of transducers is used, the diameters of the rotor and stator of the opposite type transformer increase; and the overall height of the concentric type rotary transformer increases. The existence of rotary erase heads adds to the number of channels in the rotary structure, thus increasing the overall size of the rotary transformer.
If a rotary erase head is to be avoided, with a resultant reduction in size and complexity of the mechanical system of the VTR, the head width (or gap length) of the recording heads should be matched to the track pitch formed by recording in the SP mode. In the present example, guard bands are eliminated and, thus, rotary erase heads are made unnecessary, if the gap length of the recording head is equal to the 20.5 .mu.m pitch of the SP record tracks. However, if the gap length of the record head is increased from 15 .mu.m to 20.5 .mu.m, the head overlaps a substantial portion of an adjacent track when the VTR operates to reproduce video information in the LP mode. Indeed, an entire adjacent track may be picked up when the head scans a target, or desired track.
Although adjacent tracks are recorded in both the SP and LP modes by heads which exhibit different azimuth angles, thus relying on the phenomenon of azimuth loss to reduce crosstalk interference picked up from an adjacent track during reproduction, it is known that crosstalk suppression due to azimuth loss is not perfect. Such crosstalk suppression is quite effective if only a portion of an adjacent track is picked up, but the carrier-to-noise (C/N) ratio of the reproduced signal, which is an indication of crosstalk suppression, decreases as the head overlaps a greater portion of the adjacent track. If the head overlaps the entire width of the adjacent track, as may occur if the gap length is 20.5 .mu.m and the track scanned by the head in the LP mode has a width of 10.25 .mu.m, the C/N ratio is sufficiently deteriorated as to make crosstalk interference a significant problem. Thus, if tape speed in the SP mode is twice the tape speed in the LP mode, the gap length of the recording/reproducing heads should be less than the pitch of the tracks that are recorded in the SP mode and, thus, the use of rotary erase heads cannot be easily avoided.